Impact modifiers are widely used to improve the impact strength for thermoplastics and thermosets with the aim to compensate their inherent brittleness or the embrittlement that occurs at ambient or sub zero temperatures, notch sensitivity and crack propagation. So an impact modified polymer is a polymeric material whose impact resistance and toughness have been increased by the incorporation of micro domains of a rubbery material. This is usually done due to the introduction of sub microscopic rubber particles into the polymer matrix that can absorb the impact energy or dissipate it. One possibility is to introduce the rubber particles in form of core-shell particles. These core-shell particles that possess very generally a rubber core and a polymeric shell, having the advantage of a proper particle size of the rubber core for effective toughening and the grafted shell in order to have the adhesion and compatibility with the thermoplastic matrix.
The performance of the impact modification is a function of the particles size, especially of the rubber part of the particle, and its quantity. There is an optimal average particle size in order to have the highest impact strength for a given quantity of added impact modifier particles.
These primary impact modifier particles are usually added in form of powder particles to the thermoplastic material. These powder particles consist of the agglomerated primary impact modifier particles. During the blending of the thermoplastic material with the powder particles, the primary impact modifier particles are regained and are dispersed, more or less homogenously, in the thermoplastic material.
While the particle size of the impact modifier particles is in the range of nanometers, the range of the agglomerated powder particles is in the range of micrometers.
Agglomeration during the recovery can be obtained by several processes, as for example, spray drying, coagulation or freeze drying or combination of spray drying and coagulation techniques.
It is important to have a powder that has good flow ability and does not generate dust. The generation of dust depends largely on the particle size distribution of a powder. Dust is especially generated from the small particles present in the sample. Dust should be avoided for at least two reasons: for health reasons and for avoiding dust explosions. A powder with has a large portion of coarse powder, on the other hand, is more difficult to introduce in the polymer matrix and to regenerate the initial impact modifier particles homogenously distributed in said matrix.
It is, as well, important to have an impact modifier powder that has no negative influence on the thermoplastic polymer. As negative influence, it is understood, for example the color stability, the thermal stability, the hydrolysis stability of the thermoplastic polymer comprising the impact modifier, either on function of the time or the temperature or both.
All these influences might occur due to the architecture of the core-shell but more particularly the impurities and side products employed during the synthesis and treatment of the impact modifier powder. Usually, there is no special purification step of the impact modifier, just a separation of solid versus liquid. Therefore more or less important quantities of any chemical compound (impurities, by-products) employed are still incorporated in the impact modifier. These chemical compounds should not influence the thermoplastic material in a major way as for example degradation of optical and/or mechanical and/or rheological properties with time and/or temperature and/or hygrometry.
It is also, as well important, to separate the impact modifier from the reaction medium in the easiest way, meaning essentially the use of less of resources as possible. As resources can be seen equipment involved, energy, and more generally utilities and any products.
One objective of the invention is to obtain an agglomerated impact modifier powder that has an average particle size in the range of micrometers a small ratio of fine power and a small ratio of coarse powder.
Another objective of the invention is a process to prepare an agglomerated impact modifier powder that has an average particle size in the range of micrometers a small ratio of fine power and a small ratio of coarse powder.
Still another objective of the invention is to prepare a agglomerated impact modifier powder with a volume average particle diameter smaller then 500 μm, while the amount of small particles (having average particle diameter smaller than 20 μm) is smaller than 5%, preferably with an amount of small particles smaller than 2%.
The value of the volume average particle size alone is not sufficient, as it includes no information about the particle size distribution. Therefore other values as the value at certain cumulative percentages are needed in order to have information concerning the distribution.
A still further objective of the invention is to have a fast, simple, efficient and economical process to synthesize the impact modifier and separate it from the reaction medium without further purifying and unnecessary washing steps.
Objective of the invention is to reduce waste water and cut down utilities, especially energy cost and capital investment for equipment involved in the separation.
A further objective of the invention is to have thermoplastic composition containing an impact modifier that has a good compromise between all the properties of the impact modified thermoplastic polymer as having high impact strength, while not reducing the viscosity of the polymer composition and color change at elevated temperatures, due to the influence of impurities or by-products used during the preparation of the impact modifier.
Synthesis of core shell impact modifiers and the coagulation of latexes are well known in state of the art.
The document U.S. Pat. No. 5,534,594 describes the synthesis of butadiene based core shell impact modifiers. The cited preferred isolation methods after synthesis are spray drying or coagulation, but no details are given; neither on the methods nor on the characteristics of the powders.
The document U.S. Pat. No. 5,514,772 describes a method for producing powdery and granular polymers. The method includes two coagulation steps: coagulating a latex of the polymer, the latex containing a sulfuric acid ester series and/or sulfonic acid series anionic surface active agents, with an acid to an amount of the polymer of 40 to 80 weight percent at a first coagulation step, and then bringing coagulation to completion with an acid or salt at a second coagulation step. A powder with a sharp particle size distribution is obtained. The process described involves four vessels and the pH is always below 3.
The document U.S. Pat. No. 4,897,462 describes a slow coagulation process in a pH range from 2 to 6 during the coagulation, followed by a fast completion of the coagulation at a pH<2, in order to obtain a spherical powder without fine particles. This slow coagulation is achieved due to addition of salt in diluted concentrations meaning a large amount of water.
The document EP0985692 describes a core-shell impact modifier composition comprising an intermediate sealer stage of an crosslinked alkyl acrylate in order to obtain a powder with an average slurry particle size smaller then 300 microns after coagulation. However no information about the ratio of fine particles is given the average particle size is given only. Additionally no influence of the impact modifier on the characteristics of the composition can be found.
WO2006/057777 describes the synthesis of core-shell MBS particles by emulsion graft polymerization. The polymer particles are coagulated with sulfuric acid and further heated up to 85° C.
WO 2009/126637 describes functional MBS impact modifiers synthesized by a multistage emulsion polymerization. At the end the reaction mixture obtained is coagulated in order to separate the polymer. The coagulating treatment is performed by bringing into contact the reaction mixture with a saline solution (calcium chloride or aluminum chloride—CaCl2 or AlCl3) or a solution acidified with concentrated sulfuric acid and then to separate, by filtration, the solid product resulting from the coagulating, the solid product then being washed and dried to give a graft copolymer as a powder.
WO2009/118114 describes an impact modified polycarbonate composition with a good combination of color, hydrolysis and melt stability. The rubber core is based on polybutadiene. For the preparation of the graft rubber polymer salts of fatty acids, especially of carboxylic acids are used. The yellow index of the compositions given at 206° C. is quite important: 20 or higher.
The document EP 0900 827 describes emulsion grafted polymers, and especially MBS core-shell polymers, which are substantially, free from components that degrade the thermoplastic polymer in that case polycarbonate. Therefore such components are omitted during the synthesis. More generally in order to increase the thermal stability of the impact modified polycarbonate any basic compound has to be avoided during the synthesis of the impact modifier. It is especially insisted on the use of a certain class of surfactants during the emulsion polymerization, especially sulfate- and sulfonate-containing surfactants.
The document WO2008/014464 describes polymeric core-shell nanoparticles with an interphase region and their use for rubbers. The nanoparticles are used to improve the moldability and tenacity of the rubber compositions. These nanoparticles are no impact modifiers as they are used for rubbers.
All prior art is silent about an gradient polymer in the impact modifier and generally silent about the particle size distribution of the agglomerated impact modifier particles and especially the fine content in connection with the recovery or isolation process of the powder after the synthesis especially its pH and choice of electrolyte and the performance of such an impact modifier in a thermoplastic composition.